This invention relates to an improvement in a passivating coating or collar applied to metal oxide varistors (MOVs) which are primarily zinc oxide and which are useful in a valve block of a surge arrester.
Surge arresters are used to protect electrical equipment against insulation damage resulting from voltage surges which exceed the normal operating voltage of the equipment. Typically, an arrester includes a valve section and a gap section in series inside a cylindrical insulating housing. The valve section is made up of one or more metal oxide, primarily zinc oxide, varistor disks stacked face-to-face. Each of the disks of the arrester is generally provided with a passivating coating material about the periphery, or collar, to improve its stability via preventing flashover, surface conduction, corona effect, tracking, etc. under high electrical surges (e.g., switching transients, lightning, etc.) and temporary overvoltages. The passivating material is generally a ceramic material coated on the peripheral surface of the disk to provide a ceramic collar material. The ceramic based collar material must provide:
1. isolation of the MOV surfaces from the immediate environment and prevention of gas or particle diffusion;
2. high mechanical strength and/or elastic properties;
3. higher resistivity than the MOV valve block at maximum continuous operating voltage;
4. similar dielectric constant to that of zinc oxide (i.e., .apprxeq.8.5 for MOV at extremely high electrical stress in the upturn region of the current voltage characteristic);
5. identical thermal properties with zinc oxide (i.e., thermal/heat conductivity, thermal expansion, heat diffusivity, specific heat, etc.); and
6. compact adhesion and nearly no variation in the physical gap between the collar and the MOV surfaces at any stress level.
U.S. Pat. No. 4,046,847 discloses the production of metal oxide varistors which may be incorporated into an over-voltage surge arrester either alone or as one of a number of arrester valve disks. A ceramic slurry may be coated on the peripheral surface of the disk prior to one of the reheating cycles used during the sintering process so that it will set in the course of reheating to form a flashover preventive collar. This patent discloses that a suitable slurry for this purpose is a water-based one containing a dry weight ingredient unit of filler-clay mix, of which 80% is mullite and 20% is Florida kaolin. The filler-clay mix is combined with 10% dry weight of an inorganic binder consisting of equal weights of monoaluminum phosphate and concentrated phosphoric acid. The combination is slurried with about 60% dry weight of water as a vehicle. The slurry is applied by spraying the disk to a thickness of about 1/4 millimeter. The slurry will cure or set to form a ceramic collar at anywhere above 250.degree. C., depending upon the time at that temperature. The disclosure of this patent is incorporated herein by reference.
U.S. Pat. No. 3,959,543, the disclosure of which is incorporated herein by reference, discloses a sintered zinc oxide, non-linear resistance surge arrester disk provided with a glass anti-flashover collar having a relatively low fusing temperature and a relatively low coefficient of thermal expansion. The glass comprises the following constituents in proportion to one another by weight: about 44.5% lead oxide; about 24.5% zinc oxide; about 20.5% boric oxide; about 4.5% silicon oxide; and about 6.0% cupric oxide. In making the disk, the zinc oxide composition in powdered form is pressed into a disk shape with a diameter of about 3.4 inches and a thickness of somewhat over 1 inch to form a blank. This blank is then sintered by firing in air at a temperature of about 1200.degree. C. for about five hours. Thereafter, it is cooled at about 100.degree. C. per hour, and a slurry of finely divided glass particles, having the composition as described above and mixed with ball clay to hold the glass in suspension, is applied to the outside surface of the perimeter of the disk. The disk with the applied slurry is then fired again in air at a temperature of between 640.degree. C. and 650.degree. C. for about 30 minutes to cause the glass particles in the slurry to fuse to one another and to the disk to form a collar.